Corrosion-resistant ferrous alloys



Patented Sept. 12, 1939 \Jl Ubb HGIUI UHUU PATENT: OFFICE CORROSION-RESISTANT FEBROUS' ALLOYS Herbert Henry Uhlig, Cambridge, Mesa, assignor to The Chemical Foundation, Incorporated, New York, N. Y., a corporation of Delaware No Drawing. Application May 15, 1937,

' Serial No.142.829

15Claims.

This invention relates to the production of corrosion resistant ferrous alloys, more particularly to ferrous alloys having a special surface by reason of which they are rendered peculiarly resistant to pit corrosion.

, The availability and realtive cheapness of iron, coupled with the amenability of the iron-carbon system to heat treatment and working, with a consequent advantageous modification of .its physical properties has established this metal as of basic importance in the technological fields. A major disadvantage of iron and the older ferrous alloys was' their ready susceptibility to corrosion. The problem of rendering iron resistant to corrosion has always been the major problem of the industry.

.As a broad proposition, the problem of increas-.

ing the inherent corrosion resistance of iron' and its alloys has usually been approached from the same angle; namely, by adding to the iron a quantity of nobler or more passive metal or metals in amounts sufilcient to impart their general characteristic of corrosion resistance to the iron. Consonant with this idea, in recent years, there have been developed a large number of ferruginous alloys which present a greatly increased resistance to general corrosion. This group of alloys can broadly be defined as iron base alloys which contain metal alloying ingredicuts in sufficient quantities to substantially pass'ivize the iron. Examples of such commercial alloys are the iron-chromium series, such as the rustless irons and steels, the ferromolybdenum alloys, the ferrotungsten alloys, and the like.- 85 Of these, the important and classic example is the 18-8 stainless steel. V

with the advent of these'stainless steels, it was thought that the optimum iron base alloy had been found. Such steels displayed a most 40 satisfactory resistance to general corrosion or surface oxidation. However, as the use of these steels was extended, and particularly to those fields where the corrosive environment was particularly drastic, it was found that they were sus- 45 ceptible to other and more insidious forms of corrosion, namely, intercrystalline corrosion and pit corrosion.

Intercrystalline corrosion, as is known to those skilled inthe art, is a form of corrosion which is 50 engendered and developed along or adjacent to the grain boundaries of the alloy. It was generally concluded that this form of corrosion'was caused by the depletion of chromium in the area of the grains adjacent the grain boundaries by i being taken up by the carbon to form carbides.

However, recent advances in the field have successiully overcome this particular dimculty. These passive ferrous base alloys, and particularly the stainless steels, may now be rendered "substantially immune to intercrystalline corro- 5 sion by proper control of the carbon content and the heat treatment, or by the addition of alloy constituents such as titanium, or columbium, which present a preferential afllnity for the carbon, thus etfectively precluding the removal of 10 the passivizing chromium from the grains.

In the present state of the art, therefore, a single major problem of corrosion remains to be solved. This is the problem of pit corrosion;

Pit corrosion is a peculiar and particularly it dangerous type of attack. The actual result of the attack is the formation of a series of segrev gated pits which progress perpendicularly into the body of the steel, ultimately producing a type of honeycomb structure. 20

A considerable amount of research has been done on this particular problem. The investigators in the field have not been agreed as to the precise cause of that attack. The initiation of pit corrosion has been attributed to a variety of 25 factors; such as, carbide inclusions, differential strains in the surface of the metal, electrochemical potentials set up by reason of differential metallic constituency, the presence of ferrite caused by cold work, the presence of adhered 30 incrustations of foreign matter, and the like.

As explained more fully in the copending application of John C. Wulff, Serial No. 141,921 filed May 11, 1937, the-whole problem of pit corrosion has been broadly and thoroughly investigated. In this investigation, each of the factors which might have an important or potential bearing upon the initiation of pit corrosion was separately investigated. 1

It was determined as a result of the investigation that the effective cause of pit corrosion was the mechanical imperfections in the surface of the steel established as a result of the cold working. These imperfections, for the most part taking the form of declivities, cracks, pits, and the like, appear to serve as the loci for the initiation of pit corrosion, after which the attack became continuous and self-sustaining, due, apparently, to the establishment of local oxidation-reduction electrolytic cells.

As explained in the prior application referred to, potentially passive iron alloys, of the type defined, may be rendered substantially immune to pit corrosion -by subjecting the steel article, preferably after the final stage of cold working, to a 55,

ZOZ. bUlVll'UOi l IUHO;

ici

vacuum anneal and quenching. It was there exremoved and non-metallic inclusions in the form of oxides, sulphides, and the like, were broken down and largely eliminated. Tests made upon steel so treated showed a substantial immunity of the steel, and the articles made therefrom, to this type of corrosive attack.

Concurrent investigations, dealing with the same problem, have lead to another and effective method of protection. This method in a broad sense involves the same utimate result, but such result is achieved by-a different type of treatment. Considered briefly, the present method involves the protection of potentially passive. ferrous alloys against initiation of pitting by chemically resurfacing the article so as to eliminate the mechanical imperfections of the surface, thus to present a continuous smooth surface free from potential foci of pit corrosion.

In this novel process of protection, a special and particularly effective chemical resurfacing bath or solution was developed. This particuar bath, while most effective, is here given, and is to be considered as an illustrative embodiment of the broad principle of the treatment.

As was fully explained in the earlier application, it was concluded as a result of a long series of experimentsthat initiation'of pit corrosion is largely due to macroscopic and submacroscopic surface imperfections in the steel produced as a result of the cold plastic deformation. It appeared that these mechanical surface imperfections serve as, so to speak, minute reservoirs for the accumulation and stagnation of the saline medium which is the effective pit corrodent. In these circumstances, it is probable that electrochemical actiqn ensues in which chlorine ions of the corrodent react with the metallic constituents .of the alloy and form soluble products. These soluble corrosion products are chemical reducing agents, hence the protective film in the localized area is incapable of reformation, due to the lack of oxygen. This area then becomes the focus of attack and a pit or cavity is formed. An oxidation-reduction cell is then formed between the metal on the walls of the pit and that outside the pit. The oxygen concentration outside the pit being higher than that inside, the attack is accentuated. w

As will be appreciated, and as is more fully explained in the copending application, the cold working of the metal tends to establish a mechanically imperfect surface, the imperfections of which are composed of depressions or declivi- I face are further accentuated by the presence of macroscopic or submacroscopic nonmetal inclulished and maintained between the present method of. chemical resurfacing of a potentially pittable steel and the .older and superficially analogous methods of pickling. In the present method, as presently will. appear, a strong acid is employed for the sloughing or dissolving action. A high acid concentration is advisedly chosen, 'and its chemical activity sedulously increased by utilizing it at elevated temperatures so as to insure a positive and appreciable dissolution of the actual surface metal, and this whether or'not such surface does happen to contain some oxide. In the older pickling methods, for the most part, a dilute acid is employed for the purpose of preferentially attacking the surface oxides with the minimum possible attack on the metal. Indeed, most of the commercial pickles contain certain colloid adjuvan-ts or addition agents which act as inhibitors and tend to prevent the action of the acid on the metal itself. In other words, in the conventional pickling treatment, actual attack on the metal is sedulously avoided, in sharp contradistinction to the present method where it is advisedly sought and attained.

Similarly, the present method, in its funda mental concept, is carefully to be diflerentiated from electrolytic pickling. In such pickling process, relatively high acid concentrations may be used. However, in these pickling treatments, the work to be cleaned is made the cathode of the circuit and a very high current density is impressed for the avowed purpose of obtaining a copious evolution of hydrogen. The function of the acid is largely to provide an adequate supply of hydrogen and the action of the hydrogen is mechanical, for as it expands, it pries off the oxide scale. In these electrolytic pickles, no attempt is made to dissolve the metal. In most operations, as a matter of fact, the metal body itself is more or less automatically protected.

Thus, as is usually the case, lead anodes are used,

, tween the present method of resurfacing ferrous alloys to reduce their susceptibility to pit corrosion and the bright dip used for cleaning nonferrous articles.

The present method of chemically resurfacing passive alloys which are susceptible to pit corrosion is equally to be distinguished from earlier methods of passivizing alloy steels. In such passivation treatments the steel is subjected to the.

action of a passivizing agent, such as a solution of nitric acid or chromium trioxide, for the purthe pose of developing a film (usually conceived to be an oxide film) which resists general surface corrosion. In such treatment the surface of the metal is built up, so to speak, and no positive attempt is made to dissolve off sections of the surface metal, for, as a matter of fact, the essential function of the passivizing agent is to render the metal surface more resistant to the dissolving action of the solutions.

The major function of the chemical resurfacing treatment, of the present invention, is to remove the areas which form the foci of pitting.

Such treatment subserves other and beneficial rosion is largely engendered by mechanical imperfections, it is advisable to conduct the resurfunctions such as the removal of organic and inorganic stains, surface inclusions, adherent oxides, sulphides, and other inhomogeneities, as well as strained metal produced as a result of cold working. This treatment, therefore, proprotected i reate th a mien n ac ivity to positively attack and dissolve the meta r ar nce, as no e, pit corfacingtreatment after the laststage of fabricaation in which such imperfections may be developed. It is to be appreciated, however, that the present method of treatment is available not only for the treatment of steel articles in the fabrication process, but also after any period of use of the article. In point of fact, it is advisable periodically to treat those articles, which, in normal use, are subjected to an environment which strongly induces pitting. Since the treatment involves the simple expedient of contacting the surface with a liquid, such treatment may be carried out at any stage of the fabrication, from ingot to finished article, or at an installation site, by immersion, brush ng, spraying, or otherwise suitably applying the solvent and subsequently removing the acidic solution by washing. The treatment is likewise emcacious for the treatment of articles after forming or heating operations at the installation site. For example, the resurfacing of such passive steels is particularly recommended after soldering or welding of steel articles or after any operation which would tend to develop pit-engendering imperfections of the type defined.

To more clearly explain the invention, typical examples of preferred resurfacing solutions will be described. These, however, are considered to be illustrative of the'underlying principles involved and not as defining the only methods or exclusive means by which these are effectuated.

In developing the improved resurfacing solution, tests were conducted on stainless steels of known uniform ty. As described more fully in the copending application referred to, these specimens were produced from a single ingot whose actual composition was 18.13 Cr, 8.94 Ni, 0.08 C.

The ingot was hot rolled and samples were cut from the sheet. These samples were rolled into strips of different thicknesses and then annealed to produce a homogeneous structure and finally were pickled and cleaned.

The susceptibility to pitting was determined by utilizing the improved accelerated pit corrosion method described in the earlier application,

namely, by contacting the surface .of the specimen with an aqueous solution containing 10% of FeCl: and cc. of 2.45 NHCl per litre;

A series of specimens were subjected to varying degrees'of cold working, ranging from 2% to 50% reduction so as positively to establish mechanical imperfections on the surface. This differential working also produced a corresponding amount of ferrite. c c c When tested by the accelerated corrosion method, these cold-work specimens displayed a marked susceptibility to pit corrosion. In the the immersion treatment.

case of severely worked specimens, the pitting was rapid and extensive.

An equivalent series of cold worked samples were then made up and treated in a special resurfacing solution. This solution comprised parts by weight of commercial hydrochloric acid, parts by weight of commer su 1) ur 0 ac d, 5.5 parts by weight of titanium te rac e, and 24.5 parts by weight 0 wa er. 11 making up the solution, a portion of the hydrochloric acid was diluted with an equal amount of water and this was slowly added to the titanium tetrachloride. The solution was rapidly agitated until all of the intermediate reaction products were dissolved. The remainder of the formula was then added.

This solution was heated to a temperature between '70 C. and 80 C. and the cold worked specimens, of proven pit susceptibility, were immersed and held there from about two to five minutes or more. The treated specimens were then washed and the surface scrubbed.

The specimens were then subjected to the accelerated corrosion test for four hours. At the end of the period, examination showed a substantially complete absence of pitting. These experiments were carried out numbers of times, using specimens which had been cold worked to different degrees. In each case, including the most severely worked samples, the resurfacing treatment markedly reduced pitting in FeCla.

Microscopic examinations of the specimens before and after the resurfacing treatment definitely show that the mechanical imperfections of the worked surface are removed.

A series of tests was conducted to check the conclusion that the treatment does actually resurface the steel. In these tests, the specimens were carefully dried and weighed before and after The results proved that during the treatment an appreciable quantity of metal was physically removed from the surface. Chemical analysis of the solution itself, after a period of use, confirmed this finding. While, as will be appreciated. the amount of metal removed depends on a number of variables, such as the temperature and concentration of the solution, the condition of the work. and the like, it is found as a general rule that under the conditions given above, the surface of the specimens was calculated to have been reduced approximately .00044 an inch. It appears that this period of treatment, under the conditions'of acid concentration and temperature mentioned, is sufficient to slough off the strained and scarified surface approximately down to the base of the deeper declivities or, so to speak, to the declive level. This is to say, that the resulting surface, while not in the strictest sense a microscopically flat, plane surface, is a substantially fiat surface, free from those spicular projections and sharp declivities which tend to facilitate the accumulation and stagnation of the saline corrodent. In a mechanically analogous sense. the chemical resurfacing may be conceived as a type of chemical erosion of the sharp projections of the walls of the striae, scarifications, or other declivities, to produce a type of flat, undulating surface which minimizes or precludes the entrapment and ac-- cumulation of the pit corrodent or corrosionengendering media. Whatever may be the actual role that such a resurfacing area plays in respect to the corrodent, it is found that the directed treatment does, in fact, reduce pit susceptibility of the treated steel in a very striking manner.

The special chemical resurfacing treatment 75 ucts may be employed for the fabrication of 10 Jewelry.

The major function of the treating solution, as

has already been noted, is to positively dissolve a portion of the surface metal. This function is surserved by the acidic constituents. The titanium tetrachloride appears to serve a number ene c a unc ions. In the first place, it plays an important, if indefinable,'role in imparting to the roduct the in ch ract g or sheen. In the chemical system, its important go scram appears to be in displaced from the acid as a result of the dissoution of the metal. In this respect it has been noted that the apparent evolution of hydrogen is considerably less than that which would be ex- 23 pected in view of the acid concentrations used,

indicating that the tiitanium reacts with the nascent hydrogen. It s also particulary .0 e

observed that even with the substantial dissolution of metal, and concomitant hydrogen dis- 3o placement, there is no evidence of hydrogen embrittlement of the treated surface.

It will be appreciated that the described method of treatment maybe employed in conjunction with other methods for modifying the chemical or physiochemical characteristics. Thus, improved products are obtained by subjecting steel articles to the described pit inhibiting treatment and then submitting the article to a passivation treatment, as, for example, by immersion in a 40 solution of nitric acid or chromium trioxide. Since the. initial treatment in fact resurfaces the work and provides a chemically clean surface, the effect obtained from the passivation is considerably accentuated. a

This desirable result of inhibition of pit corrosion may be obtained by a treatment with other types of baths. For this purpose, a small quantity of a passivizing agent such as nitric acid may be added to the main bath. Articles have 50 been treated in a bath of this character having the following formula:

v Per cent Tick, 5.5 H01 29.5 H2SO4- 40.0 HNO: 0.5 1120. 24.5

ype of treatment is the same as that heretofore described. The articles may be rendered 60 high in luster and markedly resistant to pit corrosion by immersion in such a solution, which is maintained at a temperature of from 70 C. to 80 C., for a period of from approximately two to five or more minutes. v

65 It will be appreciated that the formulae given ,herein are indicative of the general type of solution to be employed. Manifestly, the relative concentration of the acids 'in the formula may be varied. Similarly, the percentage of titanium 70 tetrachloride may be varied over a considerable range. It has been found that concentrations of from 4% to 7% by weight of the final solution insures the desired improved results. The length of time to which a particular article is subjected to the resurfacing solution likewise may vary detaking up the hydrogen,

pending upon the previous history of the article, in other words, the amount of resurfacing that is necessary to secure the desired pit inhibition.

It will be seen, therefore, that the described treatment most beneficially modifies the steel, more especially in markedly increasing its resistance to pit corrosion. While it has been pointed out that pit susceptibility of these passive ferrous alloys is considerably increased by cold working. it is clearly to be understood that this 10 is not here presented as the sole cause of the initiation of pitting. It is found that adhered or included inhomogeneitiesof the surface, such as inclusions of slag, oxides, sulphides and the like,

' portant bearing inthe initiation of the cracks,

checks and fissures, for it is to be expected that under stress, such mechanical imperfections would initiate at the heterogeneous interface. Again, it is known that the adhesion of foreign matter on the surface of-the metal, during use, tends to induce pitting by contact corrosion. Therefore, when mention is made of the effects of cold working and inhomogeneities, it is to be understood that this is intended to comprehend the correlated as well as the individual effect of these factors. The treatment described, as will be appreciated, serves not only to dissolve or slough oi the mechanically imperfect metal, but so also to remove the inhomogeneities in the metal. When the article is periodically treated, in the manner described, the resurfacing solution similarly functions to remove extraneous adhesions on the surface which would tend to 'induce pitting.

It will be understood, therefore, that the particular solutions which have been described are given as examples of any equivalent solvent which subserves the major function of the in- 40 vention, namely, the chemical resurfacing of the potentially pittable steel to a degree to substantially eliminate the mechanical and chemical foci of pitting and consequently to markedly reduce the susceptibility of the steel to pit corrosion. I

I claim: a

1. That method of treating articles composed o ferrous alloys, containing passivizing alloy components, to render such articles resistant to pit corrosion, which comprises, subjecting the containing substantially 50% of a mixture of sulphuric and hydrochloric acids, and maintaining the article in contact with the solvent for a period of time sufficient to insure the removal of surface metal to the order of 0.0004 in. to insure a submacroscopicallysmooth surface substanso tially free from pit-engendering mechanical imperfections.

2. That method of diminishing the pit corrosio'n susceptibility of articles composed of passivized ferrous alloys which comprises, treat- :5 ing the'articleafter the final stage of mechanical working, with highly active metal solvent and for a period of time sufficient to dissolve a sumcient amount of surface metal to substantially eliminate the mechanical imperfections which form pit-engendering fool the said highly active solvent having a dissolving action on steel equivalent to that of an aqueous solution containing substantially 50% ofa mixture of sulphuric and hydrochloric acids.

3. That method of diminishing the pit corrosion susceptibility of articles, composed of passivizedferrous alloys which comprises, treating the articles, after the final stage of cold working, with an acid solvent comprised of over 50% of a mixture of sulfuric and hydrochloric acids and for a period of from substantially 2 to 5 minutes to dissolve an appreciable quantity of surface metal to establish a smooth surface, free from inhomogeneities and mechanical imperfections at which pitting initiates.

4., A chemical bath for the treatment of steels which comprises, a strongly acidic'solution containing over 50 per cent. of strong mineral acids.

which actively dissolves the steel together with titanium tetrachloride and substantially .5% of nitric acid. 1 5, A chemical bath for the treatment of ferrous alloys which comprises an aqueous solution of over 50 per cent. of a mixture of sulphuric and hydrochloric acids and titanium tetrachloride. 6. A chemical bath for the treatment of ferrous alloys to render such alloys resistant to pit corrosion, which comprises, an aqueous solution containing substantially 30 parts of hydrochloric acid, substantially 40 parts of sulphuric acid, and substantially 5 parts of titanium tetrachloride.

7. A method of treating articles composed of ferrous alloys which contain passivizing alloying components, to increase the resistance of such alloys to general surface corrosion and pit corrosion, which comprises, treating such articles,

after cold working, with a solution including a I I highly active metal solvent containing over 50% of a mixture of sulphuric and hydrochloric acids and substantially .5% of a passivator of the type of nitric acid, and maintaining the article in contact with the solution for a period of time suflicient to insure the removal of surface inhomogeneities and to insure the dissolution of suflicient quantity of the surface metalto substantially eliminate the mechanical imperfections in the surface developed as a result of the cold working. v

8. A method of treating articles composed of ferrous alloys which are susceptible to pit corrosion, to render such articles substantially resistant to pit corrosion, which comprises, treating such articles, after any type of cold work which would induce mechanical. imperfections, with an aqueous solution containing substantially ,30 parts of hydrochloric acid and substantially 40 parts of sulphuric acid, and maintaining the 'acid in contact with the article until suillcient surface metal has been removed to establish a submacroscopically smooth surface substantially free from pit-engendering mechanical imperfections.

' 9. A method of treating stainless steel articles to markedly increase their resistance to. pit corrosion, which comprises, immersing the article in an aqueous solution containing a preponderant Q l .24: in

amount of strong mineral acids and a smaller amount of titanium tetrachloride, the mineral acids being in'sufliclent concentration to actively attack and dissolve the steel, and maintaining the article in contact with the solution for a period of time suflicient to insure the dissolution of appreciable quantities and of the order of 0.0004 in. of surface metal and the establishment of a submacroscopically smooth surface from which pit-engendering mechanical imperfections have substantially been eliminated.

10. An improved stainless steel article characterized by a marked resistance to pit corrosion, which has been chemically resurfaced in acidic solution containing over 50% of a mixture of sulphuric and hydrochloric acids after the final stage of cold working.

11. A chemical bath for the treatment of ferrous alloys, to render such alloys resistant to pit corrosion which comprises, an aqueous solution containing substantially 30 parts of hydrochloric acid, substantially 40 parts of sulphuric acid, substantially 5 parts of titanium tetrachloride and substantially 0.5 part of nitric acid.

12. A {stainless steel article of marked resistance to pit corrosion which has simultaneously been chemically resurfaced and passivized by treatment in a bath containing over 50 per cent. of strong mineral acids consisting of sulphuric and hydrochloric acids and less than 1% of nitric acid.

13. An austenitic chrome-nickel steel characterized by a marked resistance to general corrosion and pit corrosion which has simultaneously been chemically resurfaced and passivizedby treatment in a bath containing over 50 per cent. of strong mineral acids consisting of sulphuric and hydrochloric acids and less than 1% of nitric acid.

- 14. A method of treating steel articles which are susceptible to pit corrosion to render such articles more resistant to such corrosion, which comprises, periodically contacting such articles, while in service, with a strong metal solvent comprising an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids and maintaining the contact for a period of time suflicient to remove surface inhomogeneities and adherent matter and to dissolve off surface metal to the order of 0.0004 in. to thereby eliminate pit-engendering mechanical imperfections of the surface.

15. A method of treating steel articles which are susceptible to pit corrosion to render such articles substantially resistant to such corrosion, which comprises, treating the articles with an aqueous solution containing over 50% of a mixture of sulphuric and hydrochloric acids and substantially 5% of titanium tetrachloride for a period of from substantially 2 to 5 minutes.

HERBERT HENRY UHLIG. 

